This invention relates to large dynamoelectric machines and, in particular, to an improved wedging system for retaining conductor bars in stator core slots.
Large dynamoelectric machines such as electrical generators employ a laminated stator core for transmitting induced voltages to the generator terminals through stator conductor bars. The cores are usually made by assembling already-slotted punchings or laminations into an annular housing for later containing the generator rotor. The slotted punchings, when assembled, define axially-extending radial slots which terminate at the radially inner-circumference of the stator annulus. The stator bars, or conductors, are laid in the radial slots and a wedging system is used to hold the bars in place against electromagnetic forces present when the machine is operating. If the wedging system is not effective, conductor insulation may be damaged in the ensuing vibration, ultimately leading to a forced outage of the generator.
One example of a generator wedging system is shown in U.S. Pat. No. 3,139,550 to Geer and assigned to the assignee of the present invention. That patent describes, in detail, the application of a pressure wedge and a "herringbone" wedge to a stator slot for retaining the conductor bars in place in the stator slot. The herringbone portion of the wedge is used to discourage the herringbone wedge from "backing out" of the slot. The patented Geer wedge system cannot compensate for radial settlements of the stator bars in the stator core slots which occur after an extended period of dynamoelectric machine use. According to the present invention, this is because there is a lack of flexibility in the wedging system to accommodate residual shifting of the stator bars. The wedges may be retightened but this is a time-consuming major overhaul. Hence, there is a clear need within the industry to produce a stator wedge which will provide a residual tightening or follow-up effect to accommodate subsequent bar movement and settling.
Certain ripple spring configurations have been used in combination with the aforedescribed patented wedging system, but it has been found that any improved result is also diminished over time because of temperature creep within the ripple spring system. Such ripple springs have been positioned along the sides of the conductor bars so as to inhibit movement in the radial direction and ground the coils in the slots. U.S. Pat. No. 3,665,576 to Nordmann and Schmidt describes the problems associated with wedging systems and further suggests a wedge and associated hydraulic pump for applying such a wedge so that sufficient residual force is available after aging to avoid bar movement. The patent does not suggest the use of a residual spring force but rather advocates the use of an "over-force" for applying stator wedges to the stator bars. The Nordmann et al. patent discusses not only the importance of applying an initial overforce so as to leave residual pressure after aging, but also the importance of knowing how much overforce is being applied so as to insure its adequacy while avoiding the problem of broken parts. There is, then, a clear need to be able to calibrate the force which is applied by the wedging system to the conductor bars. Moreover, there is a need to know how much residual force is left applied to the conductor bars in order to determine its adequacy.
Since the environment of an electrical generator is relatively compact, there is a need to keep the mechanism for effecting construction or repair relatively simple and uncomplicated. Since there are hundreds of such stator slot wedges contained within a typical generator, it is important that the construction and method of applying slot wedges be as economical, effective, and non-time-consuming as possible.